1. Field of the Invention
The present invention relates to an image forming apparatus having a function of determining the type or the like of a recording target medium.
2. Description of the Related Art
In an image forming apparatus, various types of sheets as recording target media are used such as plain papers, thin papers, and cardboards. As a way of classifying the sheets, the sheets are divided into several types of groups depending on the grammage, which is the weight per unit area of the sheet, and optimum printing conditions may be set for each group. Image forming conditions to form an image such as the speed of carrying a recording medium, the transfer voltage used for transferring a toner image, and the optimum temperature for fixing a toner image by a fixing device, are adjusted in accordance with the thickness of the sheet.
Typically, a user selects a group of classified sheets, for example, from a control panel of the image forming apparatus. The types of sheet can be roughly divided into plain paper and cardboard. The cardboard may also be further divided into three to four types.
Recently, an image forming apparatus has been proposed in which a sensor to determine the paper type (hereinafter referred to as a medium sensor) is installed and which does not need user's manual operation for the selection of a paper type.
The material characteristics of a sheet to be detected by the medium sensor are material characteristics other than grammage, such as thickness and light transmittance. At present, there is no proper sensor that can directly detect grammage and that can be mounted on an image forming apparatus.
Generally, since grammage is printed on a package of sheets, it is a measure that is easy to use when the paper type is to be identified by the user. However, the medium sensor is to identify the paper type by a different measure such as thickness or light transmittance, and therefore it has a problem that misidentification easily occurs.